1. Field of the Invention
The present invention relates generally to a continuous casting technic. More specifically, the invention relates to a method and apparatus for continuously performing compressive forging for cast steel derived from a continuous casting process.
2. Description of the Background Art
In the conventional art, it has been regarded in inevitable to form central segregation in a continuously cast steel. This segregation is caused by condensation of carbon (C), sulfur (S) and phosphorus (P) in the molten metal near the central axis of the cast steel during the cooling and solidifying process. Such segregation degrades the cast blocks. Particularly, in case of thick steel plate, such segregation in the cast steel may degrade the mechanical properties by causing stratification or layering lamination.
Segregation in cast steel is caused at the final stage of solidification due to the solidification shrinkage or bulging of the solidifying shell which draw the condensed molten metal to the solidifying end and results in central segregation.
In order to eliminate central segregation in the cast steel, various techniques have been attempted. For example, one technique attempted to electromagnetically stir the metal in the secondary cooling zone. However, such attempts failed to completely eliminate segregation at the semi-micron level and therefor are not yet satisfactory.
On the other hand, an in-line reduction method, in which the solidifying end is compressed during the solidification period by means of a pair of rollers has been proposed in "Iron and Steel" Vol. 7, 1974, pages 875 to 884. In this in-line reduction method, it is also required to compress the solidifying block during the stage where the solidifying block contains a relatively large proportion of unsolidified steel. If the force of this compression is not sufficiently great, cracks can form at the interface between the solidified steel and the still molten portion. On the other hand, when compression at the aforementioned solidifying stage is excessive, inversely segregated areas in which certain components of the desired alloy are missing can be created at the center of the cast steel during the compression process.
In order to avoid the aforementioned defects, the Japanese Patent First (inexamined) Publication 49-12738 discloses a method for compensating for reduction of volume of the solidifying cast steel by reducing gaps between pairs of rolls. On the other hand, the Japanese Patent First Publication (Tokkai) Showa 53-40633 discloses a method for performing heavy compression by means of a casting die at the end stage of solidification. The improvement for the method of Tokkai Showa 53-40633 has been proposed in the Japanese Patent First Publication (Tokkai) Showa 60-148651, in which electromagnetic stirring is performed, or ultra-sonic waves are applied to the solidifying steel during the solidification. This process along with substantial compression by means of the casting die during the solidification stage helps to reduce segregation.
However, in the former case as disclosed in Tokkai Showa 49-12738, bulging and other defects cannot be completely avoided even when pairs of rolls are provided to reduce the gaps between them as several mm/m. In addition, in this case, when the position of the rollers is not appropriate, the light compression process may actually degrade the cast steel by creating worse segregation around the center. On the other hand, in the later case, heavy compression by means of the casting die may cause internal cracks of the solidifying steel and generate inversely segregated areas. However, the improvement in the semi-macro segregation can be achieved, this method requires quite delicate adjustment of the compression conditions. Namely, when the heavy die compression is performed at a stage in which a relatively large proportion of unsolidified steel exists, it is possible to create cracks at the interface between the solidified section and the unsolidified section. Still worse, if the heavy die compression is applied while a relatively large proportion of unsolidified metal is left, an inversely segregated area can be formed. On the other hand, if such compression is performed at a stage when an excessively small proportion of unsolidified metal is left, compression is not so effective in avoiding segregation. By performing electromagnetic stirring or by applying ultra-sonic waves, centerline segregation can be reduced by increasing the uni-directional crystalline orientation. However, it is still not satisfactory for avoiding creation of the centerline segregation and so forth for a wide range and variety of thicknesses, casting speeds, temperatures and so forth encountered when forming a steel block.